With advanced development and wide utilization of power energy resources, assessing good or bad condition of a power system becomes increasingly important. For instance, parameters such as voltage and current are the most important reference for solar cells, zinc-air batteries and the like. By measuring variations of the voltage and current of the power system in different loads, an I-V curve can be obtained and served as an important reference to observe energy consumption and element characteristics of the power system.
In the past, measuring current-voltage characteristics of the power system mainly uses resistor as a load. By changing resistance the voltage-current characteristics of the power system in different loads can be obtained. However, to do measurement by changing different resistors takes a lot of time. Moreover, variable resistor generally cannot withstand temperature effect caused by great current. This limits measurable power system specifications. Moreover, the number of resistors increases rapidly with accuracy demand but becomes difficult to realize on smaller volume of loads.
Since variable resistor load is difficult to implement in practice, some conventional techniques try to get variable loads by incorporating resistors with analog circuits, or employing capacitor charge and discharge approach. However, the two approaches mentioned above need a basic duty frequency which restricts the degree of sampling frequency taken by users. This is because the sampling frequency must be much lower than the basic duty frequency of a simulated load so as to ignore impact of modulation of the basic duty frequency.
U.S. Pat. No. 4,456,880 entitled “I-V Curve Tracer Employing Parametric Sampling” employs a switched-capacitor resistor to do charging and discharging, and digital sampling of output voltage and current. However, using the switched-capacitor generates a basic duty frequency in the system. U.S. Pat. No. 5,512,831 entitled “Method and Apparatus for Testing Electrochemical Energy” employs a parallel field effect transistor (FET) as a load, and through a digital feedback approach to control output current of a measurement system. The digital feedback frequency is the basic duty frequency of the system.
In order to implement a real variable resistor load and overcome the constraint of the basic duty frequency, the present invention employs an R-2R resistor network to realize the variable resistor load and an operational amplifier incorporating with a power transistor to perform analog feedback control. As the analog feedback control does not need sampling, there is no basic duty frequency and the related bandwidth limitation, hence a greater stable range can be achieved.